Washington — When economist Gerald Barney was lecturing in China recently, he urged that China set itself the goal of making the Yellow River, traditionally muddy with eroded soil, run clear. He says he then saw his listeners ''slump in their seats , because they recognized that this is a major undertaking.''
As principal author of the Carter administration's Global 2000 Report, Dr. Barney is concerned with soil erosion worldwide. China's problem reflects the global problem. Soil erosion threatens to destroy croplands that feed and clothe humanity. Yet effectively combating it will be a mammoth and costly enterprise.
Despite China's extensive program of terracing fields and reforesting hillsides, Dr. Barney explains, the intensive farming essential for feeding 1 billion people has resulted in ''the soil content of the Yellow River increasing 50 percent since 1948.'' With silt raising the river bottom 2 inches and pushing the river's delta another mile into the Gulf of Chihli every year, ''I had to tell them it is like China bleeding, that China cannot expect a prosperous future unless it saves its soil.''
Dr. Barney says he believes China will make a major conservation investment and will succeed in controlling its soil-loss problem.
''Any people who could build the Great Wall and go on the Long March can make the Yellow River run clear,'' he says. Nevertheless, at this time of strained budgets worldwide, just how much various countries can invest, or are willing to invest, to protect the world's food-producing soils is not clear.
Experts agree that any protective measures are likely to be extremely expensive. However, the hard data necessary to estimate that cost country by country does not exist.
Only the United States has carried out the detailed resource surveys necessary to calculate the cost of stabilizing its basic agricultural resources. The US Department of Agriculture puts the figure at $103 billion over the next 50 years. That would more than double the current $1 billion annual rate of spending on measures to reduce farm-related damage to farmlands, forests, and water resources.
On the basis of this $2 billion-a-year estimate to care for the 1.3 -billion-acre US ''agricultural land base,'' equal treatment for the world as a whole could run well over $20 billion a year. Not only does the world have an estimated 13 billion acres of farmland, rangeland, and forests, but proportionately this land is more subject than US farmland to the overgrazing, overfarming, and deforestation pressures that lead to severe environmental problems.
Among indications of the stresses felt outside the US are the wholesale loss of forests and the spread of deserts, which together can ultimately leave once-productive land useless for agricultural purposes.
According to United Nations estimates, the world's 2 billion acres of desert could become 7.5 billion acres by the end of the century if present trends continue. Similarly the world's forests, which already have shrunk from 11.9 billion acres in 1950 to an estimated 6.4 billion in 1978, could cover only 5.4 billion acres by the year 2000.
Both the loss of forests and the growth of deserts most seriously affect third world countries. Many countries in Africa are hit the hardest, and they can least afford the fight to stop the process.
Clearly swift action is vital if world food shortages actually are about to escalate dramatically. Action is also urgent if irreversible damage is indeed being done to cropland, forests, and fisheries that will be needed at some time in the future.
However, if serious food shortages are not just around the corner and if the world, in fact, has a surplus of potential agricultural land, then investment can be postponed until the world returns to easier economic times.
ooking 20 years into the future, forecasters typically foresee a 6 billion-plus world population. In the year 2000, this population is expected to be growing by 100 million people a year, compared to the 75 million-a-year rate of 1975. Forecasts generally agree that 90 percent of the growth will take place in the underdeveloped world.
Dr. Barney says such population projections impose an immediate need to conserve the world's limited supplies of land and water. He insists that the world must set conservation goals for itself or rapidly exhaust its basic resources.
As evidence that motivation and public awarness bring results, he points to the Netherlands and Israel. These two countries are reclaiming land from the sea and from the desert, he says, because both governments and people ''recognize that without proper and continual protection, their productive land will disappear very quickly.''
Dr. Barney acknowledges that it is difficult to build public support for costly conservation measures where land is less visibly threatened.
Building dikes to protect man-made Dutch fields from the North Sea or building stone cairns to harvest overnight dew for a desert orchard in Israel may be relatively easy to understand and support. It seems far harder to talk taxpayers and governments into spending money to protect currently productive agricultural land from numerous threats, including:
* Overintensive farming, which leaves soil vulnerable to relatively gradual wind and water erosion. This can ultimately turn the land into desert.
* Overuse of irrigation, which can leave the soil waterlogged, build up the salt content, or alkalinity, and exhaust water supplies.
* Air pollution and acid rain, which can reduce crop yields.
* Loss of a stable water supply as rainwater-storing forests are cut down.
* The gradual spread of housing, industry, and highways into farming areas.
Dr. Barney says governments must step in to correct the situation. ''When our market system is dealing with something like soil that requires centuries to be replaced,'' he says, ''the government must act, because the market will not take the long-term interests of society into account.'' Since market forces cannot provide any short-term return to justify soil-conservation investments, he says, governments must ''find some way to give farmers an economic incentive to protect the world's soil resources we all depend on.''
Edouard Saouma, head of the UN Food and Agriculture Organization (FAO) also says protective action is urgently needed. Addressing the Committee on Food Aid Policies and Programs in Rome this spring, Dr. Saouma said: ''Hunger is spreading. Emergency situations are multiplying. The poor countries are exhausting themselves to import just enough food to survive.''
Dr. Saouma pointed out that, although total world food production increased 2 .4 percent in 1981, serious food shortages are building in certain parts of the world.
''The situation in Africa continues to be particularly worrying,'' he said, explaining that ''food production rose by barely 2 percent - or well below the 3 percent growth in population.''
FAO officials also point to Africa's disappearing forests and dwindling fish catches as warning signs that population pressures are reaching dangerous levels.
Forests are stripped to provide building materials, firewood, or cropland. Typically, individual farmers are left with farms too small to return adequate incomes. Adding to the strain on a developing economy, rising oil prices reduce a country's ability to plant, fertilize, and harvest crops or fish its waters, because these activities now depend increasingly on oil-derived fuels and chemicals.
FAO figures show that an overall increase in world grain production masks sharp regional differences. Caught in a cost-price squeeze, 19 developing countries show a decline in total grain production; another 18 are experiencing a drop in per-capita cereal output. Developing nations are in an increasingly precarious situation, the FAO reports, because the world's reserve grain stocks have dropped to 14 percent of annual world consumption.
To free developing countries from depending on dangerously low world grain reserves, held primarily in the US, the FAO study, ''Agriculture: Toward 2000,'' concludes that third world farm output must more than double between 1980 and 2000. The FAO estimates, however, that it would take an investment of from $110 billion to $150 billion a year to achieve this rate of agricultural growth.
Rudy Dudal, director of the FAO Land and Water Development Division, acknowledges that large investments are needed to protect and improve the world's agricultural base. ''But when the world spends between 1 and 2 billion on armaments every day, to say there is no money is just not correct,'' he says. ''It is just a matter of setting priorities.''
Dr. Dudal warns that obtaining funds for saving the world's soils ''may be like with other catastrophies - that people will act on it when it is a bit late.'' This would be a costly blunder, he says: ''If there is an earthquake, you can rebuild your city, but if you destroy your land, you cannot restore it, at least at a reasonable cost. The cost would be so much higher to restore land than to preserve it.''
Tomorrow: New incentives to save US soils